It can be used in aggro-sensitive fights to avoid getting aggro.
Holy Nova generates no threat. However, it will put you on the aggro table, so if no other player is attacking a mob, the mob will attack you.
Its mana cost is very high, so avoid spamming it if you will need to heal after you hit it a few times.
Holy Nova was originally a 31-point holy talent, and has been subject to many tweaks over time, including changes in cooldown, mana cost, and actual effect.
In sovjet Russia non elites holy nova you !